There are some excellent novels set in virtual worlds – both social and gaming worlds – providing an easy introduction to the terminology, concepts and possibilities.
Pygmalion’s Spectacles was possibly the first novel about a virtual world. A good, if short, read.
Huckleberry Hax writes fiction mostly set in Second Life. He exposes both the lighter and darker sides of virtual worlds.
The Alterworld series offers a very readable – and enjoyable – introduction to game-orientated worlds. The series also tackles some interesting concepts in terms of ‘what is real’, ‘can the virtual become real’ … without ever becoming technical or unreadable.